Coronal mass ejections - dynamic evolution in the heliosphere

The Sun is an active star and its most violent activity phenomena are known to be flares and coronal mass ejections (CMEs). CMEs may abruptly disrupt the continuous outflow of solar wind and expel huge clouds of magnetized plasma into interplanetary (IP) space with velocities of a few hundred to a few thousand km/s. When directed at Earth, their embedded magnetic fields and their preceding shocks compress and distort the Earth`s magnetic field and may cause geomagnetic storms. Our modern life is very sensitive to the effects of CMEs. Especially fast and massive CMEs may cause failures in spacecraft navigation/communication systems and moreover, induce currents leading to blackouts of power plants on the ground. The changing conditions of our near Earth space and their consequences are commonly known as "space weather". Due to the increased need and economic dependence on space technology it becomes more and more relevant to enhance the knowledge about our space environment, and to reliably forecast our space weather. Single spacecraft observations image the extended three-dimensional CME structures projected as 2D objects in the plane of the sky. Hence, the geometry of CMEs and their true propagation direction and velocity can not be derived. The unique NASA mission Solar Terrestrial Relations Observatory (STEREO), launched in October 2006, consists of two almost identical spacecraft (STEREO-Ahead and STEREO-Behind) that continuously separate from each other. This enables us for the first time to study the 3D properties of CMEs. Moreover, STEREO is equipped with an optical instrument suite (SECCHI) that may follow CMEs all the way from the Sun to Earth, and even beyond. The goal of the project is to seamlessly study the evolution of CMEs and their interaction with the ambient solar wind flow over the entire distance range Sun - Earth. To get conclusive results we intend to perform in addition to detailed single event studies a statistical analysis of CMEs that occurred during the minimum and rising phase of the present solar cycle. The results are crucially important to improve models that are used to forecast the arrival times of CMEs at Earth. Due to the lack of observations in the inner heliosphere, a systematic study of the behavior of particular model parameters in reliance on observations could not be performed yet. With STEREO we are able to do this important step.

We live in the neighbourhood of an active star, the Sun. Solar activity may present itself quite dynamic, which can be observed as energetic outbursts (flares) as well as mass ejected from the solar corona (coronal mass ejections: CMEs). CMEs are huge clouds of magnetized plasma, which are expelled from the Sun into interplanetary space having velocities of a few hundred to a few thousand km/s(!). If traversing our Earth, due to their embedded magnetic fields and their preceding shocks, CMEs may trigger geomagnetic storms, which may cause failures in spacecraft navigation/communication systems and moreover, induce currents leading to blackouts of power plants on the ground. The project, Coronal mass ejections dynamic evolution in the heliosphere, contributed important results towards a better understanding of the physical processes of the evolution of CMEs and their impact on the Earths atmosphere. This will enable us to respond more effectively and efficiently to the major challenges of space weather confronting society today. Using the unprecedented observational data from the NASA mission STEREO, we intensively studied the propagation behaviour of CMEs in interplanetary space. By comparing the ambient solar wind flow, using analytical as well as numerical models, and the observed kinematics of CMEs, we found that the solar wind strongly influences the evolution of CMEs. We derived that high-speed solar wind streams were able to accelerate CMEs at far distances from the Sun. On the other hand, slow CMEs, propagating ahead of a fast one, were found to cause an abrupt deceleration of the fast ejection. Under certain conditions, CMEs showed to clear the way for a subsequent ejection by depleting the plasma density in interplanetary space. The results show the need of more deep investigations for determining the solar wind plasma parameters in interplanetary space. Knowing the structure of the background solar wind will deliver crucial information, finally enabling us to forecast CME arrival times at Earth. The impact of CMEs on the Earths atmosphere was statistically analyzed for a large number of events covering the time range 2003-2010. From this we obtained strong correlations between density enhancements in the Earths thermosphere and magnetic field strength as well as speed of a CME. By actually measuring the CME characteristics around 1 hour before their impact, we may estimate the effect of orbital decay of satellites due to the increased thermospheric density. These results will also support physical models for predicting space weather disturbances in the upper Earths atmosphere, to further improve their reliability and accuracy.